TRANSACTIONS OF THE SECTIONS. 3 



If this condition (of the evanescence of certain terms,) is not ad- 

 mitted, the integration cannot proceed directly as in those investiga- 

 tions. But Mr. Tovey has shown that in this case, i. e. when those 

 terms are finite, certain forms will still be solutions : and these are pre- 

 cisely those for elliptically polarized light. — (Journal of Science, No. 71.) 

 It also appears from that paper and the author's (Phil. Trans. 1838, 

 Part II.) that the non-evanescence of those terms is the characteristic of 

 elliptic vibrations, as their evanescence is of rectilinear, and this implies 

 the unsymmetrical or symmetrical arrangement of the molecules of 

 aether as referred to the direction of the ray. 



The discussion between Mr. Tovey and Mr. Lubbock (Journal of 

 Science, December 1837 and January 1838,) turns upon the proposition 

 involved in Fresnel's theory, " That every system of molecules (consti- 

 tuted as here supposed) has at every point three axes of elasticity : and 

 that if these be taken as axes of co-ordinates, the evanescence of the 

 terras above referred to is a necessary consequence." This proposition 

 is essential to the whole investigation of the wave-surface ; and thus it 

 would follow, that in all media we may assume the axes so that this 

 condition is fulfilled. 



This then appears irreconcileable with the characteristic of elliptic 

 vibrations before laid down. The author has followed up the subject 

 in a paper which will appear in the Philosophical Transactions. 



Remarks on M. Daguerre's Photogenic Process. 

 By Fox Talbot, Esq., F.R.S. 



The first part of M. Daguerre's process consists in exposing a silver 

 plate to the vapour of iodine, by which it becomes covered with a 

 stratum of iodide of silver, which is sensitive to light. Mr. Talbot 

 stated that this fact had been known to him for some time, and that it 

 formed the basis of one of the most curious of optical phsenomena, 

 which, as it did not appear to have been observed by M. Daguerre, he 

 would describe to the meeting. Place a small particle of iodine, the 

 size of a pin's head, on a plate of silver, or on a piece of silver-leaf 

 spread on glass. Warm it very gently, and you will shortly see the 

 particle become surrounded with a number of coloured rings, whose 

 tints resemble those of Newton's rings. Now, if these coloured rings 

 are brought into the light, a most singular phaenomenon takes place ; 

 for the rings prove to be sensitive to the light, and their colours change, 

 and after the lapse of a short time their original appearance is quite 

 gone, and a new set of colours have arisen to occupy their places. 

 These new colours are altogether unusual ones ; they do not resemble 

 anything in Newton's scale, but seem to conform to a system of their 

 own. For instance, the first two colours are, deep olive green, and deep 

 blue inclining to black, which is quite unlike the commencement of 

 Newton's scale. It will be understood that the outermost ring is here 

 accounted the first, being due to the thinnest stratum of iodide of silver, 

 furthest from the central particle. The number of rings visible is 

 sometimes considerable. In the centre of all, the silver-leaf becomes 



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